Increasing the discharge capacity of electrochemical cells is an ongoing objective of manufacturers of electrochemical cells and batteries. Often there are certain maximum external dimensions that place constraints on the volume of a given type of cell or battery. These maximum dimensions may be imposed through industry standards or by the amount of space available in the device into which the cells or batteries can be put. Only a portion of the volume is available for the materials necessary for the electrochemical discharge reactions (electrochemically active materials and electrolyte), because other essential, but inert, components (e.g., containers, seals, terminals, current collectors, and separators) also take up volume. A certain amount of void volume may also be necessary inside the cells to accommodate reaction products and increases in material volumes due to other factors, such as Increasing temperature. To maximize discharge capacity in a cell or battery with a limited or set volume, it is desirable to minimize the volumes of inert components and void volumes.
Conventional battery cell designs incorporate a single open ended prismatic or cylindrical cell can and one matching cell end cap, used to hermetically seal the cell's internal components from the outside world. The construction and design of the cell's end cap and the manner in which it mounts to the cell's can directly effect how the cell is “activated,” or internally saturated with electrolyte, how the cell vents gas during an unsafe high pressure event, and how the cell's internal active materials are connected to its external power terminals.
A cylindrical cell is typically activated by first saturating the cell's internal components with electrolyte and then assembling the end cap to the can. Attempts to create a robust hermetic seal between the cell's can and the cell's end cap after the cell has been activated are complicated by the presence of electrolyte. This becomes especially true when using a welding process at this seam. Conventional cylindrical battery cell design avoids this problem by using non-welding techniques, such as crimping, to seal the end cap to the can after electrolyte fill. These crimping techniques are not an efficient use of cell volume and reduce the total energy capacity of a cell.
Conventional prismatic cell designs create a hermetic and volumetrically efficient weld joint between the end cap and can before activating the cell. Activation in a prismatic cell is typically achieved by saturating the internal components with electrolyte introduced through a small opening in the sealed end cap, called a fill hole. After activation is complete, this fill hole is then hermetically sealed by various means. In welded cell designs, the task of hermetically sealing the fill hole is challenging. This seal is typically achieved by the addition of parts as well as some sort of curing adhesive or an additional weld, resulting in a protrusion over the fill hole that has to be managed during cell usage. Additionally, this fill hole is typically placed off center to give central placement priority to the power terminal. In volumetrically efficient cell designs, the wall thickness where this fill hole exists is often very thin, making sealing even more challenging. The result is a highly uncontrollable, unreliable, and in-the-way fill-hole seal.
Electrochemical cells are capable of generating gas, during storage, during normal operation, and, especially, under common abusive conditions, such as forced deep discharging and, for primary cells, charging. Cells are designed to release internal pressure in a controlled manner. A common approach is to provide a pressure relief mechanism, or vent, which releases gases from the cell when the internal pressure exceeds a predetermined level. Pressure relief vents often take up additional internal volume because clearance is generally needed between the vent and other cell or battery components in order to insure proper mechanical operation of the mechanism.
A cylindrical cell is vented using a complex valve designed to initially cut off current flow when a certain internal pressure is reached and then ultimately open when the cell experiences a higher internal pressure threshold. When the valve actuates, the cell is usually considered unusable. Vent mechanisms in cylindrical cells tend to be “hidden” under the battery terminal so that they take up less space on the end cap. In addition to using up valuable cell volume that could otherwise be used for cell capacity, this results in a series of small vent “windows” in the end cap that are designed to allow gas to escape from during a high pressure event. Often, when a cell experiences this type of event, materials other than gas try to escape from the cell through this vent and end up clogging these windows. This defeats the purpose of the vent, preventing gas from escaping, and the cell ends up reaching critical internal pressures and often explodes.
Venting in a prismatic cell occurs for the same reasons as in a cylindrical cell, but is usually less of mechanism and more of an area of increased mechanical stress concentration. Typical vent designs in prismatic cells are engineered holes that burst at specific pressures. Vents, if even present in prismatic cells, are typically very small by design in order to share end cap space with the fill hole and the battery terminal. These small vents can result in similar clogs and ultimately the same explosions.
Another component of electrochemical cells are current collectors. Small electrically conductive current collectors, or tabs, typically make the connections between a cell's internal active material and its external power terminals. Due to chemical compatibility and corrosion problems, these tabs are limited to a few metal types, depending on whether the tabs are on the anode (−) or cathode (+) potential of the cell. Most cylindrical cells make their cans out of a steel alloy, which forces the can to be at anode (−) potential. This allows the active internal anode material to be connected directly to the can by a simple single current collector (tab) welded to the can. In typical cylindrical cell design, the active internal cathode material is then connected to the power terminal on the end cap. Typically, the end cap is a complex and composite design made from both aluminum and steel.
Typical battery cell features contained within a conventional prismatic battery end cap include a fill-hole that allows for the cell's activation during the manufacturing process; a valve that allows the cell to vent gas during an internally unsafe high-pressure event; and a power terminal that allows the cell to transfer power to the outside world.
Improvements to address these and other limitations of conventional cylindrical and prismatic batteries are desired.